JUST WATCHED

Will the Supreme Court say 'I do?'

MUST WATCH

Will the Supreme Court say 'I do?'07:48

After years of struggle on both sides of the issue, the question of same-sex marriage goes before the U.S. Supreme Court this week. People were already lining up outside the court Friday morning for the limited number of seats available Tuesday and Wednesday, when the justices will hear oral arguments on two cases.

For those of you who won't be in the courtroom, here's a look at what to expect.

As Joe Biden might say, it's a big family deal

These are no ordinary laws before the Supreme Court: They represent battles over a generations-old concept of marriage and the rights of an increasing number of families in legally recognized gay and lesbian relationships.

JUST WATCHED

DOMA's effect on same-sex couples

MUST WATCH

JUST WATCHED

Same-sex marriage push in New Jersey

MUST WATCH

Same-sex marriage push in New Jersey05:04

There are an estimated 120,000 legally married same-sex couples in the United States. Nine states and the District of Columbia now allow same-sex couples to marry, and a dozen others recognize "civil unions" or "domestic partnerships" that grant some of the same benefits without full marriage rights.

But 29 states have added bans on same-sex marriage to their constitutions, including California, the most populous state. Supporters of the same-sex marriage ban in California argue that it's not just tradition but also biology: "The two sexes are different to the core, and each is necessary -- culturally and biologically -- for the optimal development of a human being," Rutgers University sociologist David Popenoe has written.

It starts back in 1993, when Hawaii's Supreme Court found that the state couldn't deny same-sex couples the right to marry without a "compelling reason" and sent the issue back to the state legislature. Hawaii lawmakers quickly passed a law banning gay marriage, and advocates of traditional marriage began mobilizing to fight the prospect of gays and lesbians being able to marry.

That led to the passage of the federal Defense of Marriage Act, the 1996 legislation that forbids the recognition of same-sex marriages nationwide and bars married gay and lesbian couples from receiving federal benefits.

But DOMA is becoming an orphan. Two federal appeals courts have already struck down its benefits provision; the Obama administration has refused to defend it; and former President Bill Clinton, who signed it, now calls it "incompatible with our Constitution." With the Justice Department now arguing against it, Republicans in the House of Representatives hired their own lawyers to defend the act.

The California ban, known as Proposition 8, passed by a 52-48% margin in 2008. It eliminated the right to marry that had been recognized by California courts earlier that year. Opponents of the measure challenged it in court and have succeeded in convincing federal judges at the district and appellate levels to find the ban unconstitutional.

The justices will hear oral arguments on Proposition 8 on Tuesday. On Wednesday, they will take up DOMA.

A civil right, or a political question

One the biggest issues facing the justices is whether they can -- or should -- issue a ruling that will effectively broaden the legal definition of marriage, long restricted to heterosexual couples. Backers of DOMA and Proposition 8 say it should be up to the public to make that decision, not the courts.

"Our most fundamental right in this country is the right to vote and the right to participate in the political process, " said Austin Nimocks of the Alliance Defending Freedom, a Christian advocacy group.

"We don't need the Supreme Court to take that right away from Americans of good faith on both sides of this issue and impose its judicial solution," Nimocks told CNN's State of the Union. "We need to leave this debate to the democratic process, which is working."

But California Attorney General Kamala Harris, who is arguing against Proposition 8, said voter-approved marriage bans "are simply unconstitutional." The Supreme Court has ruled more than a dozen times that marriage is a fundamental right, "and as it relates to a fundamental right, the court will hold that under the highest level of scrutiny."

"It is one thing to read the polls, which we have discussed, which show again that a majority of Americans are in favor of same-sex marriage," Harris said. "But it is more important to read the Constitution."

Same-sex marriage advocates will be watching Justice Anthony Kennedy during this week's arguments. The generally conservative Reagan appointee has authored two opinions that advanced gay rights during his tenure, striking down state laws criminalizing homosexual sodomy and striking down a Colorado constitutional amendment that forbade local gay-rights ordinances. In that 1996 case, Kennedy wrote for the court, a state can't decide that a class of people are "a stranger to its laws."

Conservative supporters of the bans have turned their eyes toward Justice Ruth Bader Ginsburg. The justice, a Clinton appointee, told a Columbia University forum in 2012 that the 1973 Roe v. Wade decision that struck down state bans on abortion short-circuited a political consensus on abortion rights.

Karl Rove, the onetime strategist for former President George W. Bush, told ABC's "This Week" on Sunday that Ginsburg may oppose a sweeping decision in support of same-sex marriage.

"What we may see is a decision here that in essence is not a 5-4 decision, but a 6-3, 7-2 that says 'leave it up to the states,' " said Rove, whose old boss once endorsed an amendment to the U.S. Constitution to ban same-sex marriages. "In fact, we could see an 8-1."

CNN legal analyst Jeffrey Toobin, the author of a lengthy Ginsburg profile for The New Yorker, said Ginsburg is likely to find the Defense of Marriage Act unconstitutional but isn't likely to knock down bans on same-sex marriage nationwide.

"She does not believe in grand pronouncements, even liberal grand pronouncements, from the Supreme Court," Toobin said.

A shifting landscape

The court is hearing arguments as a public shift toward same-sex marriage appears to be gathering speed. The proportion of Americans who support same-sex marriage has grown from around 40% in 2007 to 53% in a CNN/ORC International poll conducted last week; 44% remain opposed.

Barack Obama ran for president in 2008 as a supporter of civil unions but not same-sex marriage. In 2012, months before facing voters again, he said his thinking had shifted and that he supported marriage rights for gays and lesbians.

"I had hesitated on gay marriage, in part, because I thought civil unions would be sufficient," he said. "I was sensitive to the fact that -- for a lot of people -- that the word marriage is something that provokes very powerful traditions and religious beliefs."

Obama's Republican challenger, Mitt Romney, ran as a defender of traditional marriage, and the party's platform opposed same-sex marriages. But since November, numerous GOP figures have emerged as supporters of same-sex unions.

Dozens of high-profile Republicans -- including former party Chairman Ken Mehlman, ex-presidential hopeful Jon Huntsman and actor Clint Eastwood -- filed a friend-of-the-court brief arguing that Proposition 8 should be struck down. And earlier this month, Sen. Rob Portman of Ohio announced he is now a supporter of the freedom to marry after finding out that his son -- a Yale sophomore -- is gay.

"Eventually, as time marches on, this is a country that believes pretty squarely in marriage equality," former Bush spokeswoman Nicolle Wallace told "Fox News Sunday."